Welding is a metal-joining process wherein coalescence is produced by heating to suitable temperatures with or without the application of pressure, and with or without the use of filler metal (American Welding Society definition). Of the welding technology disciplines, a variety of welding techniques involve the generation of very high currents which are maneuvered along current path regions of altering direction. Each of these path regions, in turn, develop an electromagnetic field having directional aspects which correspond with that region. Thus, the variously derived fields, as well as any stray fields, will mutually intersect in one way or another. The more predominate of the welding procedures involving such high current paths is generally referred to as resistance welding. Resistance welding refers to a group of welding processes that produces coalescence at faying surfaces with heat obtained from the resistance of the workpiece to the flow of the welding current in a circuit of which the workpiece is a part, and by the application of pressure. Several welding categories are classified as resistance welding. The more prevalent of these categories are resistance spot welding (RSW), resistance projection welding (RPVW), and resistance seam welding (RSEW). In resistance spot welding, the coalescence at the faying surfaces is produced in one spot or nugget by the heat obtained from resistance to electric current passing through the workpieces held together under pressure by electrodes. Variations in the RSW process differ in the application of welding currents and pressure, as well as in the arrangement of the current directing circuit (secondary domain). In the latter regard, direct and indirect welding as well as parallel and series welding have been employed. Projection welding is similar to spot welding wherein coalescence is produced by the heat obtained from resistance to electric current through the workpieces held together under pressure by electrodes. The resulting welds are localized at predetermined points by projections, embossments or intersections. In resistance seam welding, leak-tight welds can be made by a series of overlapping spot welds. These are produced by introducing time, coordinated, pulses of current from rotating wheel electrodes. Roll resistance spot welding (RRSW) is similar to seam welding, being carried out with one or more rotating circular electrodes. In resistance upset welding (RUW) coalescence is produced simultaneously over the entire area of abutting surfaces or progressively along a joint of the workpieces by heat obtained from resistance to electric current through the area of contact of the surfaces. Resistance flash welding (RFW) involves a procedure wherein coalescence is produced simultaneously over the entire area of abutting surfaces of workpieces by the heat obtained from resistance to electric current between the two surfaces and by the application of pressure after heating is substantially completed. RUW and RFW are accompanied by expulsion of metal from the resultant joint. Resistance percussion welding (RPW) achieves coalescence simultaneously over the entire abutting surfaces of workpieces by the heat obtained from an arc produced by a rapid discharge of electrical energy with pressure percussively applied during or immediately following the electrical discharge.
The main process variables involved in resistance welding are welding current, welding time, electrode force, and electrode material and design. In the majority of cases, the current during resistance welding is in the range from about 5,000 to 50,000 amperes. Because of the high values of current, resistance welding machines are designed as low-voltage sources, usually in the range from about 1 to 25 volts. These low voltages are obtained typically from a step-down transformer which usually has a single-or two-turn, cast-copper, water cooled secondary winding. Power supply to the primary windings of the transformer usually is obtained from public utilities single-phase alternating-current sources. The step-down ratio of the transformer typically is approximately 100; thus the current requirements of the primary winding are lowered to reasonable values, which generally range from 50 to 500 amperes.
Alternating current of 60 Hz is used in most resistance welding installations, although, three phase frequency converters are used to supply 2.5- to 25-Hz voltage to single-phase transformers. Welding current also may be supplied by a direct-current or stored-energy source. Direct current may be obtained from various low-voltage sources, such as rectifiers, homopolar generators, or storage batteries. The energy may be stored during relatively long periods and released suddenly from capacitors, magnetic fields, storage batteries or heavy flywheels on homopolar generators. These types of power supplies eliminate large transient loads on power lines.
The duration of welding times in resistance welding installations is short, generally being in the range of from one half cycle (120th of a second) to a few seconds. In the majority of applications, the time is in the range of 5 to 120 cycles of the 60 Hz source ({fraction (1/12)} to 2 seconds).
Weld testing predominately is carried out with procedures involving the destruction of the workpieces. Statistically significant numbers of these workpieces are subjected to such testing. In general, the strength of a single spot weld in shear is determined by the cross-sectional area of the nugget in the plane of the faying surfaces. Strength test for spot welds are discussed in Chapter 12, Volume 1 of the Eighth Edition of the Welding Handbook, American Welding Society, Miami, Fla. 33135. Additional information concerning spot weld test procedures is provided, for example, by the American Welding Society, See: AWS C1.1, Recommended Practices for Resistance Welding. 
Notwithstanding these published procedures, in a great many instances, generally, the procedures for testing are generated at the plant level and will involve the destruction of a given number or percentage of welded parts produced or submitted by smaller suppliers. The test failure of one weld may lead to consideration of rejecting an entire batch of product which may involve the rejection of thousands of items. In effect, industries currently typically are striving to achieve total quality in welded parts, i.e., a 100% acceptance quality.
An endeavor to achieve this quality weld-based production necessarily involves the evaluation of the above-noted weld process parameters. Such evaluation with respect to weld current preferably will not invade the weld circuit path, but will remain quite accurate. Weld current sensing systems may be permanently implanted in the welding process, but also should provide for portability as may be employed in spot checking and set-up calibration procedures for welding systems. The operational parameter of weld current can represent an important factor in evaluating critical components of the weld system, such as electrode shapes. Particularly where large numbers of welds are involved, as witnessed in robotically maneuvered weld machines, electrode shapes may degenerate, resulting not only in unsatisfactory welds, but also may be manifested by the phenomena of electrodes sticking to the workpiece. The latter condition may result in substantial disruption of a production line.
At the present time, weld currents principally are measured utilizing a device referred to as a xe2x80x9cRogowskixe2x80x9d coil, a torus wound coil which is positioned around a current carrier of the welding device. Because of their sensitivity to motion, installation of the coils within a weld machine requires adequate strapping or mounting and the like to avoid relative movement between the current carrier and the measuring coil. The output of the coil represents a differential of current, di/dt, which must be further treated by an integrator network to achieve a value for current. These rather inconvenient and bulksome devices additionally are limited to the measurement of alternating or pulsating weld current, not being responsive to d.c. welding systems. A weld current monitor employing a single Hall device has been described in Heckendorn, U.S. Pat. No. 5,504,299 entitled xe2x80x9cResistance Welding Sensorxe2x80x9d, issued Apr. 2, 1996. This device, however, is highly sensitive to any alteration of its orientation with respect to the weld current path. Thus, any reconfiguration of the sensor device adjacent to that path results in a requirement for recalibration. Of course, such devices are unavailable for use as portable or hand-held weld current measurement instruments.
The present invention is addressed to apparatus and method for measuring high level weld currents, particularly as are encountered in the field of resistance welding. The apparatus employs Hall effect sensors with the attendant advantage of off-current path positioning, but without the otherwise present problem of position sensitivity. As a consequence , the apparatus and method enjoy the added advantage of being utilized in quite broadened measurement applications, including an incorporation of the Hall effect based networks into portable, including hand-held, instruments.
Position sensitivity essentially is eliminated through the use of at least two Hall effect sensors which are symmetrically disposed about the center location of the apparatus housing. In particular, the sensors are located within a peripheral region of the housing surrounding at least a portion of the center location. By so symmetrically mounting the sensors, their reaction to the interfering flux patterns of adjacent extraneous electromagnetic fields is accommodated for by output signal cancellation. A consistent and reliable measuring output signal corresponding with the current sought to be measured is developed by a simple summing procedure, implemented, for example, by an instrument-borne summing amplifier network. The resultant signal may be treated in a variety of ways including digitization and subsequent mathematical treatment by a process controller. However, the measuring output signal always will be linearly related to the current value sought to be measured.
In one embodiment of the invention, two Hall effect sensors are mounted at the peripheral region of the housing which are mutually oppositely disposed along a common diameter passing through the center location of the housing. In general, that center location of the housing is aligned with the center of the current path of interest.
In another embodiment, three of the Hall effect sensors are mounted at the housing peripheral region. Each of these Hall effect sensors is positioned at one of three radii extending from the center location of the housing. The radii are mutually angularly oriented at 120xc2x0 radial spacing.
Where four Hall effect sensors are utilized at the housing peripheral region, each is positioned at one of four radii extending from the housing center location and mutually angularly oriented at 90xc2x0 radial spacing.
Because of the very high current values employed in resistance welding and the resultant rather strong electromagnetic fields involved with various directional orientation aspects, the gain characteristics of the Hall effect devices may be adjusted to avoid saturation phenomenon not only by programming lower gains within the Hall components themselves, but also by angularly orienting their flux confronting faces in an off-radius manner to diminish the confronting face area available for confrontation with electromagnetic flux. The result is a reduction or attenuation of gain by a selected amount.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly comprises the apparatus and method possessing the construction, combination of elements, arrangement of parts, and steps which are exemplified in the following description.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings.